The present invention relates generally to infrared detectors and associated fabrication methods and, more particularly, to a reference bolometer and an associated fabrication method.
Infrared detectors are used in a variety of applications to provide an electrical output which is a useful measure of the incident infrared radiation. For example, quantum detectors are one type of infrared detector that are often used for night vision purposes in a variety of military, industrial and commercial applications. Quantum detectors generally operate at cryogenic temperatures and therefore require a cryogenic cooling apparatus. As a result, quantum detectors that operate at cryogenic temperatures can have a relatively complex design and generally consume significant amounts of energy.
Another type of infrared detector is a thermal detector. Thermal detectors are typically uncooled and therefore generally operate at room temperature. One type of thermal detector that has been developed and is becoming increasingly popular is a microbolometer-based, uncooled focal plane array. A focal plane array generally includes a plurality of imaging pixels, each of which includes a bolometer disposed upon a common substrate. Each bolometer includes a transducer element that has an electrical resistance that varies as a result of temperature changes produced by the incident infrared radiation. By detecting changes in the electrical resistance, a measure of the incident infrared radiation can be obtained. Since the design of a bolometer-based uncooled focal plane array is generally less complex than cryogenically cooled quantum detectors and since these uncooled focal plane arrays generally require significantly less energy than cryogenically cooled quantum detectors, bolometer-based uncooled focal plane arrays are being increasingly utilized.
Each imaging pixel of a conventional uncooled focal plane array has a bolometer that includes an absorber element for absorbing infrared radiation and an associated transducer element having an electrical resistance that varies as its temperature correspondingly varies. Although the absorber and transducer elements can be separate layers of a multilayer structure, the absorber element and transducer element may sometimes be the same physical element. In operation, infrared radiation incident upon the absorber element will heat the absorber element. Since the absorber element and transducer element are in thermal contact, the heating of the absorber element will correspondingly heat the transducer element, thereby causing the electrical resistance of the transducer element to change in a predetermined manner. By measuring the change in electrical resistance of the transducer element, such as by passing a known current through the transducer element, a measure of the incident radiation can be obtained.
In order to provide thermal contact between the absorber and transducer elements while electrically insulating the transducer element from the absorber element, the bolometer also generally includes a thermally conductive, electrically insulating layer disposed between the absorber element and transducer element. In addition, the bolometer typically includes another insulating layer disposed on the surface of the bolometer facing the substrate which serves to structurally support the other layers and to protect the other layers during the fabrication process. See, for example, U.S. Pat. Nos. 5,286,976; 5,288,649 and 5,367,167 which describe the pixel structures of conventional bolometer-based focal plane arrays, the contents of each of which are incorporated herein by reference. However, the absorber and transducer elements can be spaced apart from one another as described in U.S. Pat. No. 6,307,194, the contents of which are also incorporated herein by reference. By spacing the absorber and transducer elements, these elements can be separately optimized even though the absorber and transducer elements remain in thermal contact.
In order to provide thermal contact between the absorber and transducer elements while electrically insulating the transducer element from the absorber element, the bolometer also generally includes a thermally conductive, electrically insulating layer disposed between the absorber element and transducer element. In addition, the bolometer typically includes another insulating layer disposed on the surface of the bolometer facing the substrate which serves to structurally support the other layers and to protect the other layers during the fabrication process. See, for example, U.S. Pat. Nos. 5,286,976; 5,288,649 and 5,367,167 which describe the pixel structures of conventional bolometer-based focal plane arrays, the contents of each of which are incorporated herein by reference. However, the absorber and transducer elements can be spaced apart from one another as described in U.S. patent application Ser. No. 09/326,937, the contents of which are also incorporated herein by reference. By spacing the absorber and transducer elements, these elements can be separately optimized even though the absorber and transducer elements remain in thermal contact.
In order to further improve the performance of conventional pixel structures, each bolometer can include a reflector disposed upon the surface of the substrate underlying the absorber and transducer elements. As such, infrared radiation that is incident upon the bolometer, but that passes through and is not absorbed by the absorber element, will be reflected by the reflector back towards the absorber element. At least a portion of the reflected radiation will therefore be absorbed by the absorber element during its second pass through the absorber element, thereby increasing the percentage of the incident radiation that is absorbed by the absorber element.
In operation, infrared radiation incident upon the imaging pixel will be absorbed by the absorber element of the bolometer and the heat generated by the absorbed radiation will be transferred to the transducer element. As the transducer element heats in response to the absorbed radiation, the electrical resistance of the transducer element will change in a predetermined manner. In order to monitor the change in resistance of the transducer element and, therefore, to indirectly measure the infrared radiation incident upon the bolometer of the imaging pixel, circuitry is generally formed upon the underlying substrate. The circuitry is generally electrically connected to the transducer element via a pair of conductive paths or traces defined by or upon the legs, pillars or the like that support the absorber and transducer elements above the surface of the substrate. By passing a known current through the transducer element, the change in electrical resistance of the transducer element can be measured and a corresponding measure of the incident infrared radiation can be determined.
In addition to the imaging pixels, a bolometer-based focal plane array also generally includes one or more reference pixels. As will be described, a reference pixel is responsive, not to incident radiation, but to changes in ambient temperature and other fluctuations in the operating characteristics of the focal plane array. Based upon the output of a reference pixel, the output of the imaging pixels can be interpreted to distinguish that portion of the output that is attributable to the incident radiation upon the imaging pixel from that portion of the output that is attributable to changes in ambient temperature and other operating conditions, thereby providing a more accurate measurement of the incident radiation.
A reference pixel typically has the same general construction as the imaging pixels described above. As shown in FIG. 2, a reference pixel 10 includes a bolometer formed upon the same substrate 12 as the imaging pixels. The bolometer of a reference pixel also includes a transducer element 14 and an absorber element 16 spaced from the substrate by two or more legs, pillars or the like 18. The bolometer of a reference pixel can also include a reflector 20 disposed on the substrate so as to underlie the absorber and transducer elements. Unlike the imaging pixels, however, the transducer and absorber elements are not thermally isolated from the substrate. Instead, the transducer and absorber elements are thermally coupled to the underlying substrate such that any heat generated by the incident radiation or by electrical current flowing through the reference pixel is transferred to the substrate and, in some instances, to a heat sink or the like upon which the substrate is mounted. By extracting the heat, changes in the electrical resistance of the transducer element of the bolometer of a reference pixel will be due, not to the incident radiation or to electrical current flowing through the reference pixel, but to changes in the ambient temperature. Hence the output provided by the bolometer of a reference pixel is attributable to changes in the ambient temperature and other operating conditions.
In order to thermally couple the absorber and transducer elements 16,14 with the underlying substrate 12, the bolometer of a reference pixel 10 generally includes a thermally conductive heat sink layer 22. The thermally conductive heat sink layer is disposed between and in thermal contact with both the absorber and transducer elements and the underlying substrate. As such, any heat generated by radiation incident upon the absorber and transducer elements or by electrical current flowing therethrough is transferred via the thermally conductive heat sink layer to the substrate. Generally, the thermally conductive heat sink layer is formed of a polyimide that, as described below, also serves as a release layer that supports the absorber and transducer elements of the bolometers of the imaging pixels during the fabrication process, but that is subsequently removed to complete the fabrication process thereof in order to thermally decouple the absorber and transducer elements of the bolometers of the imaging pixels from the substrate.
In this regard, the conventional technique for fabricating a bolometer-based focal plane array that includes a plurality of imaging pixels and at least one reference pixel generally begins with the provision of a suitable substrate 12. As known to those skilled in the art, the substrate is typically comprised of silicon and includes a plurality of integrated circuits and the associated circuitry for providing signals to and processing signals that are received from the respective pixels. A passivation layer 24, such as a layer of silicon dioxide (SiO2) or the like, is deposited upon the substrate. Thereafter, the exposed surface of the passivation layer is subjected to chemical and mechanical polishing in order to planerize the exposed surface. Following the deposition and planerization of the passivation layer, the reflectors 20 can be deposited upon the passivation layer. Typically, the reflectors are deposited in locations upon the passivation layer that correspond to the eventual locations of the respective bolometers. Thereafter, a release layer 22 of polyimide is deposited. The polyimide layer and the underlying passivation layer are then etched to define openings 26 to the substrate. Thereafter, the absorber and transducer elements 16,14 as well as any insulating layers are deposited upon the polyimide layer at locations that correspond to the positions of the respective bolometers. As such, each pair of absorber and transducer elements overlie a reflector to form a respective bolometer. Along with the deposition of the absorber and transducer elements, legs, pillars or other supports 18 are typically formed within the openings defined through the polyimide layer and the underlying passivation layer in order to connect the absorber and transducer elements with the substrate. In addition to providing mechanical support for the absorber and transducer elements, electrical leads are typically defined along or through the legs, pillars or other supports to interconnect the transducer element and the circuitry carried by the substrate.
In order to complete the fabrication of the bolometer-based focal plane array, the polyimide layer that is disposed between the reflector and the absorber and transducer elements of the bolometers of the imaging pixels is removed in order to thermally decouple the absorber and transducer elements from the underlying substrate. As such, the polyimide layer can be etched, typically by a plasma etching process. During this etching process, all of the polyimide layer associated with the imaging pixels is etched. Absent preventative measures, the polyimide layer 22 that is disposed between the reflector and the absorber and transducer elements 16,14 of each reference pixels 10 would also be etched. Since the polyimide layer provides the thermal path from the absorber and transducer elements to the substrate 12 that is necessary to prevent the output of the reference bolometer from including contributions due to the incident radiation, however, the polyimide layer of the reference pixel must underlie the entire surface area of the absorber and transducer elements to provide sufficient heat sinking for the heat generated by the incident radiation.
In order to prevent the etching of the polyimide layer 22 from undercutting the absorber and transducer elements 16,14 of the reference pixel 10, the bolometer of a reference pixel also preferably includes an oxide layer 28 disposed upon the polyimide layer and underlying the absorber and transducer elements as shown in FIG. 1. The oxide layer, typically formed of SiO2 or another oxide, is generally insensitive to the etching. As such, the oxide layer protects the portion of the polyimide layer that underlies the absorber and transducer elements from etching. In this regard, upon exposure to the etchant, the etchant begins to undercut and etch away those portions of the polyimide layer that underlie the absorber and transducer elements of the imaging pixels at the same time and at the same rate that the etchant begins to undercut and remove the polyimide layer that underlies the oxide layer associated with each reference pixel. By sizing the oxide layer such that the oxide layer has a sufficiently large footprint in comparison to the absorber and transducer elements of the reference pixel, the polyimide layer that underlies the absorber and transducer elements of the imaging pixels will be completely removed prior to removing any of the polyimide layer that underlies the absorber and transducer elements of the reference pixels. Upon completion of the etching process, the outer portions of the oxide layer extend in a cantilevered fashion outwardly from the underlying polyimide layer due to the undercutting and the removal of the portion of the polyimide layer that previously supported the outer portions of the oxide layer. See FIG. 2.
While the oxide layer 28 offers some protection for that portion of the polyimide layer 22 that underlies the absorber and transducer elements 16,14 of the reference pixels 10, the fabrication process is generally not sufficiently robust for producing large quantities of bolometer-based focal plane arrays in an automated fashion. In this regard, it is generally desirable to somewhat over etch the polyimide layer to insure that all traces of that portion of the polyimide layer that previously supported the absorber and transducer elements of the imaging pixels has been removed, thereby insuring that the absorber and transducer elements of the imaging pixels are thermally decoupled from the underlying substrate. In the process of over etching the portion of the polyimide layer associated with the imaging pixels, however, that portion of the polyimide layer that underlies the absorber and transducer elements of the reference pixels may also be at least somewhat etched or undercut. As a result of any such indercutting, the reference pixel will be less sensitive and, if the undercutting is substantial, may fail since the absorber and transducer elements of the reference pixel will not be sufficiently thermally coupled to the underlying substrate 12. The oxide layer 28 that is disposed between the polyimide layer and the absorber and transducer elements of a reference pixel can be enlarged to further protect the polyimide layer associated with the reference pixel. However, the enlargement of the oxide layer can reduce the density with which the reference pixels can be fabricated. Since a bolometer-based focal plane array is desirably quite dense, further enlargement of the oxide layer is therefore disadvantageous.
Additionally, it has been observed that the plasma etching process may proceed more rapidly in the vicinity of certain features. For example, plasma etching may occur more rapidly along the edges of conductor patterns or leads as a result of radio frequency (RF) antenna effects or the like. As such, that portion of the polyimide layer 22 that underlies the absorber and transducer elements 16,14 of a reference pixel 10 may be etched undercut in those regions proximate a conductor or lead, even if the oxide layer 28 is otherwise large enough to prevent more conventional undercutting. As described above, this undercutting the polyimide layer that otherwise underlies the absorber and transducer elements of a reference pixel reduces the thermal coupling between the absorber and transducer elements and the underlying substrate 12 and correspondingly decreases the sensitivity of the reference pixel.
As shown in FIG. 2, the cantilevered portion of the oxide layer 28 that extends beyond the polyimide layer 22 of a reference pixel 10 following etching of other portions of the polyimide layer is quite thin and fragile and is therefore prone to being fractured or broken. Fragments of the oxide layer that have been broken can then move about the vacuum chamber in which the bolometer-based focal plane array is disposed and may distort the measurements or images obtained by the focal plane array. In this regard, the fragments of the oxide layer may come to rest upon the absorber and transducer elements of an imaging pixel. Although small, these fragments would increase the thermal mass of the absorber and transducer elements and may shield the absorber and transducer elements from a certain portion of the incident radiation, thereby disadvantageously altering the output otherwise provided by the imaging pixel.
As a result of the problems associated with overetching of the polyimide layer and fracturing of the oxide layer, bolometer-based focal plane arrays must generally be manually inspected prior to being placed in service. As will be apparent this manual inspection process is quite time consuming and increases the cost of the resulting focal plane array. Thus, it would be advantageous to reliably fabricate bolometer-based focal plane arrays so as to have a higher yield and thereby permit the manual inspection of the focal plane arrays to be reduced, if not eliminated. For example, it would be desirable to fabricate a bolometer-based focal plane array in such a manner that the portion of the polyimide layer underlying the absorber and transducer elements of the imaging pixels could be fully removed without undercutting or otherwise etching that portion of the polyimide layer that underlies the absorber and transducer elements of a reference pixel. In addition, it would be desirable to fabricate a bolometer-based focal plane array having more rugged reference pixels that no longer include a thin, cantilevered oxide layer that is prone to breakage which, in turn, creates fragments that can distort the resulting measurements obtained by the bolometer-based focal plane array. Further, it would be desirable to fabricate a bolometer-based focal plane array without restricting the density with which the pixels can be formed.
These and other shortcomings of conventional focal plane arrays are addressed by the reference bolometer and the associated methods for fabricating a reference bolometer and for fabricating an array of bolometers according to the present invention. Preferably, the reference bolometer is fabricated such that a thermally conductive layer underlies the detector element, i.e., the absorber and transducer elements, and is encapsulated by a protective coating. The protective coating serves to prevent the thermally conductive layer underlying the detector element of the reference bolometer from being etched during the process of etching or otherwise removing other portions of the thermally conductive layer that underlie the detector elements of the imaging bolometers. As such, the thermally conductive layer of the reference bolometer of the present invention maintains the desired thermal communication between the detector element and the substrate such that the output of the reference bolometer is unaffected by incident radiation. As described below, the methods for fabricating the reference bolometer according to the present invention are robust and should increase the reliability and yield compared to conventional fabrication processes, thereby permitting manual inspection of the resulting focal plane array to be reduced, if desired. communication between the detector element and the substrate such that the output of the reference bolometer is uneffected by incident radiation. As described below, the methods for fabricating the reference bolometer according to the present invention are robust and should increase the reliability and yield compared to conventional fabrication processes, thereby permitting manual inspection of the resulting focal plane array to be reduced, if desired.
The reference bolometer of the present invention includes a thermally conductive layer disposed on a portion of a substrate. The thermally conductive layer includes a first surface opposite the substrate and a side surface extending between the first surface and the substrate. The reference bolometer also includes a protective coating on at least the side surface of the thermally conductive layer and, more typically, on both the first surface and the side surface of the thermally conductive layer. As described below in conjunction with the method for fabricating the reference bolometer, the protective coating serves to prevent that portion of the thermally conductive layer that underlies the detector element of the reference bolometer from being etched or otherwise removed during the removal of those portions of the thermally conductive layer that underlie the detector elements of imaging bolometers of the focal plane array. The reference bolometer further includes a detector element, typically including the absorber and transducer elements, disposed on the first surface of the thermally conductive layer. As such, the detector element is in thermal communication with the substrate via the thermally conductive layer. In this regard, the thermally conductive layer preferably defines a footprint on the substrate that is at least as large as the detector element such that the thermally conductive layer underlies all portions of the detector element.
The reference bolometer of some embodiments also includes an etch stop layer disposed on a portion of the substrate surrounding the thermally conductive layer. As also described below in conjunction with the fabrication methods of the present invention, the etch stop layer facilitates the fabrication of the reference bolometer and, more particularly, the definition of the protective coating. Further, the reference bolometer can include a reflector disposed upon that portion of the substrate underlying the thermally conductive layer or the detector element.
According to one aspect of the present invention, a method of fabricating a reference bolometer is provided. According to this method, a thermally conductive layer is initially formed on a portion of the substrate. The thermally conductive layer is formed to have a first surface opposite the substrate and a side surface extending between the first surface and the substrate. A protective coating is then deposited on at least the side surface of the thermally conductive layer and, more typically, on both the first surface and the side surface of the thermally conductive layer. A detector element, typically comprised of the absorber and transducer elements, is then constructed on the first surface of the thermally conductive layer such that the conductive layer is in thermal communication with the substrate via the thermally conductive layer.
According to one embodiment, the thermally conductive layer is formed by uniformly depositing a thermally conductive layer on the substrate and thereafter removing the thermally conductive layer from selected portions of the substrate. In this regard, the thermally conductive layer is removed from selected portions of the substrate, such as those portions of the substrate that support the imaging pixels, to define a thermally conductive island associated with each reference pixel. More particularly, the thermally conductive layer is preferably removed from an annular portion of the substrate to thereby define a moat surrounding the thermally conductive island. Thereafter, the protective coating can be deposited and the detector element can be constructed upon the thermally conductive island to form a reference bolometer.
According to one embodiment, an etch stop layer is also deposited on a portion of the substrate that surrounds the thermally conductive layer, i.e., within the moat surrounding the thermally conductive island, prior to depositing the protective coating. In this regard, a reflector is also typically deposited on the substrate contemporaneously with the deposition of the etch stop layer and prior to forming the thermally conductive layer. Following the fabrication of the remainder of reference bolometer, the reflector will underlie the detector element and will be spaced therefrom by the thermally conductive layer. In embodiments in which an etch stop layer is deposited on the portion of the substrate that surrounds the thermally conductive layer, the protective coating is then deposited on both the thermally conductive layer and the etch stop layer. Subsequently, that portion of the protective coating that has been deposited upon the etch stop layer is removed, while that portion of the protective coating that has been deposited on the thermally conductive layer is preserved. As such, the protective coating preferably encapsulates both the first surface and the side surface of the thermally conductive layer. Thereafter, the detector element can be constructed on a portion of the protective coating that is deposited on the first surface of the thermally conductive layer.
In order to fabricate an array of bolometers according to one aspect of the present invention, a plurality of imaging bolometers and at least one reference bolometer are constructed with the reference bolometer constructed as described above. During an intermediate stage of the fabrication process, however, each imaging bolometer also includes the thermally conductive layer and a respective detector element disposed on the thermally conductive layer. According to this aspect of the present invention, the exposed portions of the thermally conductive layer are then etched, thereby removing the thermally conductive layer underlying the detector element of each imaging bolometer. As a result of the protective layer that encapsulates the portion of the thermally conductive layer that underlies the detector element of the reference bolometer, however, the thermally conductive layer of the reference bolometer is not etched and, instead, continues to thermally couple the detector element and the underlying substrate.
The fabrication processes of the present invention therefore provide a robust technique for reliably manufacturing a focal plane array in which the thermally conductive layer is completely removed from each imaging bolometer while the thermally conductive layer remains intact for each reference bolometer. As such, the reference bolometers will be thermally coupled to the substrate in a reliable fashion without a concern for undercutting and without having to deposit a relatively large oxide layer that could decrease the density with which the bolometers could otherwise be spaced. Moreover, the resulting reference bolometer will no longer include an oxide layer that extends outwardly in a cantilevered fashion, thereby reducing the possibility that the oxide layer might fracture into fragments that may disadvantageously impair the measurements or image captured by the focal plane array. By fabricating the focal plane array in a more reliable fashion, the yield should be greater than conventional fabrication processes and the bolometers will no longer have to be visually inspected to the same degree as conventional bolometers, thereby decreasing the time required for fabrication and the corresponding fabrication cost.